M42 timing chain

[SiGmA_X]

Afternoon folks,

My girlfriend bought a 91 318 with engine trouble. Doesn't run, nasty chain noises from the PO. I took the valve cover off and there was a LOT of chain slop, it could move perhaps 2 teeth. Appears the tensioner wasn't doing its job. I convinced it back to proper timing (BTW, is there a front of engine TDC mark, as the flywheel has no hole.. And its stock according to my service history..) and did a leakdown, #1-3 95%, #4 40%. Pulled the head, the valves are obviously bent, its going to be sent out to be rebuilt (surface, new guides/seals, replace needed valves, valve grind).

So, in regards to the chain, do I want to just put a new tensioner piston in, or should I pull the lower cover and replace the chain too? The guides have minor wear (180k on the car IIRC), so I don't think they failed...

Any input is well appreciated!

-Roman

[wolfgangstbd]

while your in there you might as well replace all of it. but you dont really have to replace the chain. i think there are a few websites that sell the service kit

[Hick]

REPLACE THE CAM GEARS!

[SiGmA_X]

I have shop cost for all parts. What all is recommended to replace? Does anyone have high res pics of good vs bad gears, because if I hadn't read about "bad gears" online, I would assume they are fine, they sure look fine.

Why not replace the chain? Is it the gears that cause the chain to lose tension? The tensioner seems to be okay, actually, but those are hard to gauge without replacing and seeing how things sound.

[Hick]

thats one thing to replace.... do it all... if the teeth on the cam gears are pointy, then they are bad. there should be a flat top about an 1/8" wide.

like this, and any gear:


tensioner rail, guide rail, lower guide, crank oil seal, tensioner, cam gears, chain.

the reason behind the engine failure is a jumped chain. only the intake valves bent?

[SiGmA_X]

Well, we'll see when the machine shop looks at the head, but I had 95-100% leak via the intake valves, and my glove on the exhaust didn't move at all. I filled the exhaust ports on the head with parts wash fluid and it stayed in, 100%, meaning they seal reasonably well. They still could be bent.

I'd like a picture and a precise measurement if you can... I suppose I could order some and see for myself, returning a gear is an easy task. I've never seen an M42 with flat top teeth, or any cam sprockets with flat topped teeth for that matter... And I've rebuilt, with all new components, a number of different chain driven BMW/MBZ motors.. Never an M42 tho.

[Petebee]

Here is a pic with some decent used ones that I just installed on an engine that I sold:



These are what the pointy, worn ones look like:



Do yourself a favor and replace it all (chain, tensioner, gears, guides, etc.).

[SiGmA_X]

I'm now considering replacing the whole motor... I found a 105k motor with 3% leakdown and 179-183psi compression for $750 shipped. Might be worth it. I'll be into the head/chain job for $850-950 easily. Bah decisions decisions..

[Petebee]

Still check the timing components on the good used engine. I purchased my car from a fastidious original owner will full documentation and 5K oil changes at 118K miles. The pointy gears are from that engine.

[SiGmA_X]

Humm. In that case, maybe we should just replace the timing components and rebuild the head. The bottom end should be good to go for a long while if oil changes were done properly, and records show they were mostly all done properly...

[Petebee]

If you have decent records the rebuild should be okay. I think the culprit generally is not replacing the tensioner every 40-50K, which leads to sloppiness in the chain which leads to worn sprockets (and can end in what you've experienced).

[SiGmA_X]

I just re-did my spreadsheet, and with all new timing components, head set, intake valves, valve job, oil change, etc, I'm looking at $908. Humm. Might be a good idea. I have a cold leakdown of around 5-10%, so I assume rings are in great shape.

[BadCab]

Be sure to check the crank gear also, my cam gears were fine when i changed them but the crank gear was pointy. Looks like the PO went the cheap route on the last tensioner/chain job. Guide rails were cracked all over too.
This job can get expensive even if you do it yourself, or not too expensive if you just want to fix whats broken. I never touched my head internals (although it did get lifted off) and i was into it way over a grand. I did the headgasket, every other gasket, seal and oring, all the gears, guides,tensioner,chain and the water pump,thermostat,rad fan sensor, and vacuum hoses. Unfortunatly i'm addicted to "while i'm in there" and still ended up finding stuff i "needed" to get "while i was in there", like exhaust manifold bolts that snapped, guide rail C-clip, that broke, etc. It didn't help that i unbolted the oil pan and didn't realize the engine had to be lifted to get it out ( i don't have a hoist....yet) so the clutch/PP,tranny seals,shifter bushes got changed also. I was at $2k easily. I don't know exactly because i refuse to add it all up. I bought the parts over a 3-4 month period so it didn't really hurt that much. The job itself kinda sucked though. Good luck, oh and the chain whine you hear when you are done is normal.
Glen

[KenC]

I don't see how the chain can slip with pointy gears. Mine are and there is NO play in the chain at all. I don't think I could get it to jump with a prybar. But then again, my guides are all almost new as is the tensioner.

[SiGmA_X]

The gears are indeed pointy, I am going to pull the front cover when I get back from the Bahamas and see how it looks.

M50 swap was a consideration. I can get a good condition M50 w/ everything for $700. Just need an oil pan and gasket. How does the driveline hold up, Mot said it won't take the additional power what so ever. I did an M10->M50 swap w/ stock driveline and it is fine 2 years later..

Be sure to check the crank gear also, my cam gears were fine when i changed them but the crank gear was pointy. Looks like the PO went the cheap route on the last tensioner/chain job. Guide rails were cracked all over too.
This job can get expensive even if you do it yourself, or not too expensive if you just want to fix whats broken. I never touched my head internals (although it did get lifted off) and i was into it way over a grand. I did the headgasket, every other gasket, seal and oring, all the gears, guides,tensioner,chain and the water pump,thermostat,rad fan sensor, and vacuum hoses. Unfortunatly i'm addicted to "while i'm in there" and still ended up finding stuff i "needed" to get "while i was in there", like exhaust manifold bolts that snapped, guide rail C-clip, that broke, etc. It didn't help that i unbolted the oil pan and didn't realize the engine had to be lifted to get it out ( i don't have a hoist....yet) so the clutch/PP,tranny seals,shifter bushes got changed also. I was at $2k easily. I don't know exactly because i refuse to add it all up. I bought the parts over a 3-4 month period so it didn't really hurt that much. The job itself kinda sucked though. Good luck, oh and the chain whine you hear when you are done is normal.
Glen

Its a lot easier for me, I have a lift and all tools hah. Also, if you want to lift the motor, you need a transverse brace, not a engine hoist. Well, if you want to do it the correct and easy way.